Method of applying luminescent coating



Patented Feb. 16, 1943 2,311,513 mz'rnon or APPLYING LUMINESCENT ooa'rmcMaurice E. Bell, Wilkinshurg, and Leo J. Berberich, Forest Hills, Pa.,assignors to Westinghouse Electric a; Manufacturing Company, EastPittsburgh, Pa., a corporation of Pennsylvania No Drawing.

Application January 17, 1940,

Serial No. 314,274

12 Claims.

The present invention relates to the method of applying a, luminescentcoating to an electrical discharge device and more particularly to adischarge device provided with a coating of fluorescent materialexcitable by invisible radiations which converts the latter into visibleradiations.

Although the present invention is applicable to cathode ray tubes, X-rayfluoroscopes, and the like, its greatest commercial adaptation lies inthe fluorescent lamp field where the converted invisible radiations thusaugment the visible radiations to produce a high efliciency lamp.

At the present time fluorescent lamps, as they are called, are wellknown in the art. In the operation of such lamps a discharge occursbetween two electrodes, which is supported by an ionized column of gasor vapor, the resonant radiations of which lie within the invisibleregion of the spectrum. By coating the lamp with fluorescent material ofdifierent composition, this material is excited by the invisibleradiations and fluoresces, thus producing visible radiations of a colordepending upon the particular composition of the material.

In order to obtain high efiiciency of light output, the thickness anduniformity must be carefully controlled. If the thickness of the coatingis. too great, the particles nearest the discharge act as a screen forthe lower layer of particles so that visible radiations from the upperexcited layer are precluded from getting out of the envelope. Again, ifthe particles are not uniformly distributed over the surface of theenvelope but are agglomerated at certain areas, dark spots appear andthe light output from the entire surface of the lamp is thus decreased.

Many methods of applying the fluorescent coating, in an effort to obtainuniformity'of distribution and thickness, are known to the art. One suchmethod is to first clean the surface of the vessel and then apply asuitable cementing composition thereto. A powdered luminescent materialis then dusted over the cementing composition while the tubular vesselis rotated. Thereafter the glass vessel is heated to a temperaturesufilcient to volatilize the cementing agent, leaving a deposit of theluminescent material. Such method, however, has not been commerciallysuccessful for the reason that uniformity of thickness and distributionof particles cannot be obtained.

Another process employed in the art is to first coat the interior of thetubular vessel with a binder comprising light transmitting enamel havinga lower softening temperature than that of the vessel itself upon whichthe particles of luminescent material are propelled, or the vesselitself is heated to the softening point and the particles propelled onthe surface so as to become embedded therein. Here again there is noassurance of uniformity of thickness nor of distribution of theparticles as such is practically impossible of attainment.

A further method now in commercial use is to mix finely divided powderwith a high viscosity binder such as plasticized nitrocellulose in amylacetate solution and spray or flush the solution on the surface of thevessel, draining off the excess. The vessel is then dried and heateduntil the binder carbonizes and again heated at a higher temperature inthe presence of oxygen to fire off the carbon. Although this methodproduces a coating more uniform in thickness and particle distributionthan any of the above noted methods, nevertheless it possesses severaldisadvantages. First, it necessitates firing at a high temperature toremove all traces of a carbonaceous residue which would otherwise poisonthe lamp during fabrication. Secondly, after removal of the carbonresidue, the coating can be very easily damaged during assembly of otherparts in the envelope, and precautions must accordingly be taken toprevent this happening.

A still further disadvantage of such prior art method resides in thefact that an additional step is required in which oxygen must beadmitted to the lamp in order to enable'firing of the carbon. Thisnecessarily requires additional time to again remove excess oxygen whichmust be done rather completely in order to obtain satisfactory operationof the completed lamp.

The primary purpose of the present invention is to provide a method forapplying a fluorescent coating which results in uniformity of thicknessand particle distribution and at the same time facilitates fabricationof the completed lamp.

To this end the present invention employs a heating, while the resins ofthe present invention leave no carbonaceous residue. Hence, aftercoating the tubing, it may be handled with ease and without fear ofdestroying the coating, thus facilitating fabrication or assembly of theelectrodes and similar parts into a completed lamp. During the exhaustof the lamp, which necessarily requires heating to a temperature withinthe depolymerizing range or the resins, in order to degasify the variouslamp parts, the monomers, dimers, trimers, etc., will leave the vacuumsystem along with the other gases which are pumped out of the lamp andmay be condensed in a suitable chamber forming a part 01. the customaryvacuum system.

These depolymerizable resins may, for example, comprise polystyrene,cycloparaflin resin (tradename Nevillite cyclopentadiene polymer(trade-name Neville G resin), coumarone-indene (trade-name Neville Rresin), and isobutylene polymers (trade-name Vistanex and Oppanol") allof which break down into monomers, dimers, trimers, etc., when heated toa temperature, as above mentioned, ranging from 250 to 500 C. It hasbeen found, however, that of the resins above mentioned, polystyreneforms a less firmly adhering coating than do the other mentioned resins.The adhesion can also be improved by the addition of a plasticizer, suchas trichlorbenzene, dibutyl-phthalate, the dimer of eoumarone-indene(trade-name Nevinol), and amyl naphthalene. In this case the plasticizeris of a material which does not volatilize during solvent removal, butdoes completely volatilize when the resin is removed. Other materialswhich may be used as plasticizers for polystyrene,

and which sublime on heating without carbonization, are camphor andnaphthalene, although the .latter may be undesirable since they tend toadhere to the vacuum system if removed during exhaust.

To apply the coating, a toluol solution of one of the resins or amixture thereof, as above noted, and containing for example from 10 to70% resin by weight, depending on the viscosity desired, is first madeup, to which finely divided fluorescent material is added and thoroughlymixed. The fluorescent material itself may be of any desired compositionsuch as magnesium tungstate, calcium tungstate, cadmium silicate, orzinc silicate, depending upon the particular color desired. Theproportion of the fluorescent material to the solution will determinethe thickness of the coating since naturally the more material inproportion to the solution, the thicker the coating; but, by way ofexample, a mixture of one gram of fluorescent material to each 4 cc. ofresin solution gives a very satisfactory coating.

It should also be noted that a mixture of the above noted resins hasseveral advantages not obtainable with either particular material alone.For example, polystyrene forms a viscous solution with only about 15 to20% resin in toluol but, as above noted, it does not adhere very well toglass. However, since polystyrene, the cycloparaflin or naphtheneresins, the cyclopentadiene and the coumarone-indene resins, aremiscible over a wide range of proportions, mixtures thereof can be madeto obtain a very satisfactory adherent coating.

The cycloparaflin, cyclopentadiene and conmarone-indene resins requireabout 60 to 70% resin to form a viscous solution in a solvent such astoluol. These resins adhere well to glass, but tend to melt and runsomewhat ii heated slowly,

although it heated rapidly this condition does not occur. It polystyreneis mixed with the cycloparaffln, the cyclopentadiene or thecoumarone-indene resins, in a solvent such as toluol containing, forexample, about three parts of the former to about one part of the latterresins, the resulting resin will adhere very well, will not run onheating, and the coating will be tough and resist damage during assemblyof lamp parts.

A still further advantage in a mixture of resins is that the amount ofresin used for making a solution of a given workable viscosity can bevaried very simply by varying the proportions of the two types oi resinused. That is to say, solutions of a given viscosity containing a highproportion oi! polystyrene and low proportion of cyclparamn resin willhave a relatively low resin content, and solutions of the same viscosityhaving a low proportion of polystyrene and a high proportion ofcycloparaflin resin will have a relatively high resin content.

This feature may be used to control the density or flufilness of thedeposit of fluorescent material on the lamp, if desired, since it isobvious that a solution of high resin content on depolymerizing willleave a less dense and more flufl'y deposit of fluorescent material thana solution of less resin content.

The admixture is then flushed or sprayed on the interior of the tubularcontainer, and. the solvent removed from the coating by drying in air,or more quickly by heating to approximately C., if desired. Upon removalor the solvent. the coating, as above mentioned, becomes hard andresistant to scratching or any other damage which may occur in assemblyof the electrodes and other parts of the lamp, thus eliminating thenecessity of precautions against damage, as required in connection withprior art methods.

After complete assembly the lamp is then connected to the vacuum pumpand exhausted in the manner well known in the lamp manufacturing art.During exhaust, as previously mentioned, the lamp is heated to atemperature ranging from 250 to 500 C. for the purpose of degasiiyingthe electrodes. Inasmuch as this heating is within the depolymerizablerange of the resin, it breaks down into monomers, dimers, trimers, etc.,which are drawn off by the exhaust system. This accordingly enablesremoval or the binder during the required exhaust step in themanufacture of the lamp; and as no carbon residue is formed since theresin is a depolymerizable one as distinguished from a decomposable one,the vacuum system is in no way impaired. The monomers, being a volatileliquid, may be readily condensed in a chamber provided for the purpose.

It is to be noted that while it has been stated the depolymerizableresins become monomers, dimers, trimers, etc., at a temperature rangingfrom 250 to 500 C., when heated in vacuum they decompose nearer thelower limitv at about 370 (2.; but when heated in air, a temperaturenearer the upper limit at about 475 C. is required. Moreover, it shouldbe noted that if some of the above mentioned resins are heated in airinstead of vacuum, they will leave a carbonaceous residue and for thatreason cannot be classified under the general term of depolymerizablehydrocarbon resins. Although there is a salient advantage in removingthe binder during exhaust, as it eliminates a processing step requiredby present methods, it is of course to be understood that suchadditional step can be employed and the binder removed by heating evenprior to exhaust,

if economy of manufacture and the formation, in some instances, of acarbonaceous residue is of no concern.

It thus becomes obvious to those skilled in the art that a method of apping a fluorescent coating is herein provided which produces a uniformthickness and particle distribution over the entire coated area.Moreover, after removal of the solvent, the bulb may be handled withoutfear of damage during fabrication or assembly of the various parts toform the lamp. After complete assembly the binder is removed during therequired exhaust step of manufacture, thus eliminatin a separate step ofbinder removal and reducing manufacturing costs of lamps of thefluorescent type.

It should also be noted that while it has been stated herein that thedepolymerizable resins break down upon heating into monomers, dimers,trimers, etc., we will for the sake of simplicity refer to the same inour claims merely as monomers.

Although one embodiment of the present invention has been shown anddescribed, it is to be understood that various other modifications ofthe same may be made without departing from the spirit and scope of theappended claims.

What is claimed:

1. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of luminescent material in a depolymerizableresin dissolved in a solvent, drying the coated container to volatilizethe solvent, evacuating the assembled device by connecting to an exhaustsystem, and heating the coated container to a temperature ranging from250 to 500 C. while subjected to the evacuating system to depolymerizthe resin into readily removable volatile units which are removed by theexhaust system without contamination of the latter leaving a residue ofonly luminescent material free of a carbonaceous deposit and having auniform thickness and particle distribution in direct adhesion with thesurface of the container.

2. The method of applyin luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of luminescent material in a solution ofdepolymerizable resin dissolved in a solvent, drying the coatedcontainer to volatilize the solvent, and evacuating the assembled deviceby connecting to an exhaust system,

and heating the coated container to a temperature not exceeding 500 C.while subjected to the evacuating system to depolymerize the resin intounits which are removed by the exhaust system without contamination ofthe latter leaving a residue of only luminescent material free of acarbonaceous deposit and having a uniform thickness and particledistribution in direct adhesion with the surface of the container.

3. The method of applying a luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of luminescent material in a high viscositydepolymerizable resin, evacuating the assembled device by connecting toan exhaust system, and heating the coated container to a temperature notexceeding 500 C. while subjected to the evacuating system todepolymerize the resin into units which are removed by the exhaustsystem without contamination of the latter leaving a residue of onlyluminescent mate rial free of a carbonaceous deposit and having auniform thickness and particle distribution in direct adhesion with thesurface of the container.

4. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of the material in a heat depolymerizableresin dissolved in a solvent, drying the coated container to volatilizethe solvent.

evacuating the assembled device by connecting to an exhaust system, andheating the coated container to a temperature not exceeding 500 C. whilesubjected to the evacuating system to depolymerize the resin intounitswhich are removed by the exhaust system without contamination ofthe latter leaving a residue of only luminescent material free of acarbonaceous deposit and having a uniform thickness and particledistribution in direct adhesion with the surface of the container.

5. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of the material in a solution of a heatdepolymerizable resin and a plasticizer, drying the coated container tovolatilize the solvent, evacuating the assembled device by connecting toan exhaust system, and heating the coated container to a temperature notexceeding 500 C. while subjected to the evacuating system todepolymerize the resin into units which are removed by the exhaustsystem together with the plasticizer without contamination of theexhaust system leaving a residue of only luminescent material free of acarbonaceous deposit and having a uniform thickness and particledistribution in direct adhesion with the surface of the container.

6. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of the material in a solution of a highviscosity heat depolymerizable resin and a plasticizer, drying thecoated container to volatilize the solvent, evacuating the assembleddevice by connecting to an exhaust system, and heating the coatedcontainer to a temperature ranging from 250 C. to 500 C. to depolymerizethe resin into units which are removed by the exhaust system togetherwith the volatilized plasticizer without contamination of the exhaustsystem leaving a residue of only luminescent material free of acarbonaceous deposit and having a uniform thickness and particledistribution in direct adhesion with the surface of the container.

7. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of the material in a solution of a mixtureof heat depolymerizable resins dissolved in a solvent, drying the coatedcontainer to volatilize the solvent, evacuating the assembled device byconnecting to an exhaust system, and heating the coated container to atemperature not exceeding 500 C. while subjected to the evacuatingsystem to depolymerize the resins into units which are removed by theexhaust system without contamination of the latter leaving a residue ofonly luminescent material free of a carbonaceous deposit and having auniform thickness and particle distribution in direct adhesion with thesurface of the container.

8. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of the material in a solution containingfrom to 70% by weight of a heat depolymerizable resin dissolved in asolvent, drying the coated container to volatilize the solvent,evacuating the assembled device by connecting to an exhaust system, andheating the coated container to a temperature not exceeding 500 C. whilesubjected to the evacuating system to depolymerize the resins intoreadily removable system without contamination of the latter leaving aresidue of only luminescent material free of a carbonaceous deposit andhaving a uniform thickness and particle distribution in direct adhesionwith the surface of the container.

9. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of the material in a solution containingfrom 10% to 70% by weight of a mixture of at least two heatdepolymerizable resins dissolved in a solvent, drying the coatedcontainer to volatilize the solvent, evacuating the assembled device byconnecting to an exhaust system, and heating the coated container to atemperature not exceeding 500 C. while subjected to the evacuatingsystem to depolymerize the resins into readily removable volatile unitswhich are removed by the exhaust system without contamination of thelatter leaving a residue of only luminescent material free of acarbonaceous deposit and having a uniform thickness and particledistribution in direct adhesion with the surface of the container.

10. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of the material in a solution containingpolystyrene and cycloparailin resins capable of depolymerizing uponheating without the deposition of a carbonaceous residue dissolved in asolvent, drying the coated container to volatilize the solvent,evacuating the assembled device by connecting to an exhaust system, andheating the coated container to a temperature not exceeding 500 C. whilesubjected to the evacuating system to depolymerize the resins into 10'volatile units which are removed by the exhaust readily removablevolatile units which are removed by the exhaust system withoutcontamination of the latterleaving a. residue of only luminescentmaterial and having a uniform thickness and particle distribution indirect adhesion'with the surface of the container.

11. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of the material in a solution containingabout three parts of polystyrene to about one part of cycloparafllnresins capable of depolymerizing upon heating dissolved in a solvent,drying the coated container to volatilize the solvent, evacuating theassembled device by connecting to an exhaust system, and heating thecoated container to a temperature not exceeding 500 C. while subjectedto the evacuating system to depolymerize the resins into readilyremovable volatile units without the deposition of a carbonaceousresidue which are removed by the exhaust system without contamination ofthe latter leaving a residue of only luminescent material and having auniform thickness and particle distribution in direct adhesion with thesurface of the container.

12. The method of applying luminescent material to the container of anelectric discharge device comprising coating the surface of thecontainer with a suspension of the material in a solution containingpolystyrene and cyclopentadiene resins capable of depolymerizing uponheating dissolved in a solvent, drying the coated container tovolatilize the solvent, evacuating the assembled device by connecting toan exhaust system, and heating the coated container to a temperature notexceeding 500 C. while subjected to the evacuating system todepolymerize the resins into readily removable volatile units, withoutthe deposition of a carbonaceous residue, which are removed by theexhaust system without contamination of the latter leaving a residue ofonly luminescent material and having a uniform thickness and particledistribution in direct adhesion with the surface of the container.

MAURICE E. BELL. LEO J. BERBERICH.

